Pregnant mare serum gonadotropin (PMSG) is a unique glycoprotein hormone with the biological activities of both the pituitary gonadotropins, follicle stimulating hormone (FSH) and luteinizing hormone (LH). PMSG is also the only gonadotropin which can presently be isolated from the blood in pure form. The proposed studies on PMSG are designed to gain information both on a hormone closely related to FSH and LH but also on the nature of a hormone as it appears in the circulation. Techniques of protein-polypeptide chemistry are employed to obtain highly purified PMSG and subunits for biochemical, biological, and immunological studies. In vivo and in vitro bioassays for FSH and LH are used to obtain structure-function correlates. In these studies, PMSG and its subunits are studied chemically and compared to pituitary FSH, LH and their subunits. The hormone from the endometrial cups and urinary excretion products are also studied. These studies are intended to give greater knowledge of the mechanism of gonadotropin action and reproductive function on the molecular level.